Summary: in this tutorial, you will learn how to use the PostgreSQL aggregate functions such as AVG(), COUNT(), MIN(), MAX(), and SUM().

Introduction to PostgreSQL aggregate functions

Aggregate functions perform a calculation on a set of rows and return a single row. PostgreSQL provides all standard SQL’s aggregate functions as follows:

  • AVG() – return the average value.
  • COUNT() – return the number of values.
  • MAX() – return the maximum value.
  • MIN() – return the minimum value.
  • SUM() – return the sum of all or distinct values.

In practice, you often use the aggregate functions with the GROUP BY clause in the SELECT statement:

SELECT column1, AGGREGATE_FUNCTION(column2) FROM table1 GROUP BY column1;

In this syntax, the GROUP BY clause divides the result set into groups of rows and the aggregate function performs a calculation on each group e.g., maximum, minimum, average, etc.

PostgreSQL aggregate function examples

Let’s use the film table in the sample database for the demonstration.

Film table

AVG() function examples

The following statement uses the AVG() function to calculate the average replacement cost of all films:

SELECT  ROUND(AVG(replacement_cost), 2) avg_replacement_cost FROM  film;

The following is the result:

avg_replacement_cost ----------------------  19.98 (1 row)

Noted that we use the ROUND() function to round the result to 2 decimal places.

To calculate the average replacement cost of the Drama films whose category id is 7, you use the following statement:

SELECT  ROUND(  AVG(replacement_cost),  2  ) avg_replacement_cost FROM  film  INNER JOIN film_category USING(film_id)  INNER JOIN category USING(category_id) WHERE  category_id = 7;

Here is the result:

avg_replacement_cost ----------------------  21.09 (1 row)

COUNT() function examples

To get the number of films, you use the COUNT(*) function as follows:

SELECT  COUNT(*) FROM  film;

Output:

count -------  1000 (1 row)

To get the number of drama films, you use the following statement:

SELECT  COUNT(*) drama_films FROM  film  INNER JOIN film_category USING(film_id)  INNER JOIN category USING(category_id) WHERE  category_id = 7;

The result shows that there are 62 drama films:

drama_films -------------  62 (1 row)

MAX() function examples

The following statement returns the maximum replacement cost of films.

SELECT  MAX(replacement_cost) FROM  film;

Output:

max -------  29.99 (1 row)

To get the films that have the maximum replacement cost, you use the following query:

SELECT  film_id,  title FROM  film WHERE  replacement_cost =(  SELECT  MAX(replacement_cost)  FROM  film  ) ORDER BY  title;

Output:

film_id | title ---------+-------------------------  34 | Arabia Dogma  52 | Ballroom Mockingbird  81 | Blindness Gun  85 | Bonnie Holocaust  138 | Chariots Conspiracy ...

The subquery returned the maximum replacement cost which then was used by the outer query for retrieving the film’s information.

MIN() function examples

The following example uses the MIN() function to return the minimum replacement cost of films:

SELECT  MIN(replacement_cost) FROM  film;

Output:

min ------  9.99 (1 row)

To get the films that have the minimum replacement cost, you use the following query:

SELECT  film_id,  title FROM  film WHERE  replacement_cost =(  SELECT  MIN(replacement_cost)  FROM  film  ) ORDER BY  title;

Output:

film_id | title ---------+------------------------  23 | Anaconda Confessions  150 | Cider Desire  182 | Control Anthem  203 | Daisy Menagerie ...

SUM() function examples

The following statement uses the SUM() function to calculate the total length of films grouped by film’s rating:

SELECT  rating,  SUM(rental_duration) FROM  film GROUP BY  rating ORDER BY  rating;

The following picture illustrates the result:

rating | sum --------+------  G | 861  PG | 986  PG-13 | 1127  R | 931  NC-17 | 1080 (5 rows)

Summary

  • Aggregate functions perform a calculation on a set of rows and return a single row.
  • Use aggregate functions to summarize data.
  • Use the AVG() function to calculate the average value in a set of values.
  • Use the COUNT() function to perform a count.
  • Use the SUM() function to calculate the total of values.
  • Use the MIN() function to get the minimum value in a set of values.
  • Use the MAX() function to get the maximum value in a set of values.